This program encompasses the work of seven projects, all relating to the mechanisms and consequences of interactions among cells with special emphasis on nerve cells, muscle fibers, synapses, and synaptic transmitters. It includes studies on the mechanisms of synaptic transmission, the development of synapses in cell culture, the identification of transmitter substances in invertebrates and vertebrates, regeneration of neuromuscular junctions, modulation of synaptic transmission, development of nerve cells and synapses during insect metamorphosis, photoreception in a simple eye, and transmission from photoreceptor to second-order visual neurons. It is the overall aim of the Program to learn about the interactions of excitable cells; the rapid interactions underlying synaptic transmission and the longer-term interactions important to neuronal development and synapse formation are being studied.